1. Technical Field
The present invention relates to a surface acoustic wave element utilizing a surface acoustic wave and especially, a surface acoustic wave element in which a phase velocity and a group velocity of the surface acoustic wave are different in directions, and a method of manufacturing the same.
2. Related Art
A surface acoustic wave device (SAW device) utilizing a surface acoustic wave (SAW) uses a surface acoustic wave element in which an interdigital transducer (IDT) is formed on a surface of a piezoelectric plate such like quartz. Conventionally, the SAW devices have been widely used for high frequency filters because their characteristics in high frequency region are excellent. The surface acoustic wave element included in the high frequency filters is mostly formed by a ST cut quartz plate.
Quartz is a single crystal of trigonal system, which is expressed by three crystal axes perpendicular each other, namely an electrical axis (X axis), a mechanical axis (Y axis) and an optical axis (Z axis). These axes are generally expressed as shown in FIG. 10. A cutting angle of a ST cut quartz plate 10 is expressed as (φ°=0°, θ°=approximately 123°, and Ψ°=0° by defining and using Euler angle (φ°, θ°, Ψ°). That is, as shown in FIG. 10, the ST cut quartz plate 10 is formed as the quartz plate whose Y′ and Z′ axes are obtained by rotating the Y and Z axes in counter clock wise about the X axis by θ°=approximately 123° and surface is in parallel with the plane formed by the X and Y′ axes. Also, the axis in the direction of the thickness of the ST cut quartz plate 10 perpendicular to a XY′ surface is defined as a Z′ axis.
A surface acoustic wave element 12 made from the ST cut quartz plate 10 is widely used for SAW resonators and SAW filters due to its excellent frequency stability because a first order coefficient of frequency temperature characteristics is zero. In the surface acoustic wave element 12 made from the ST cut quartz plate 10, as shown in FIG. 10, an IDT 14 and reflectors 16 are formed along the X axis.
Meanwhile, in a case where the ST cut quartz plate 10 is used for a SAW resonator included in high frequency oscillators, it is difficult to achieve characteristics to satisfy specifications required. That is, if the surface acoustic wave element 12 is used for SAW filters, a high Q value of such like resonators oscillating a specific frequency is not needed because the operation is required in relatively wide frequency range. However, the surface acoustic wave element 12 made from the ST cut quartz plate 10 has a second-order temperature coefficient that is a relatively large value, even though the first-order temperature coefficient is zero. Therefore, it is known that a resonance frequency of the surface acoustic wave element 12 made from the ST cut quartz plate 10 fluctuates approximately 100 ppm in the temperature range from minus 20 degrees centigrade to plus 80 degrees centigrade that is operation temperature range of the SAW resonators. Thus, frequency characteristics required in high accuracy communication equipment or the like cannot be satisfied.
Consequently, the inventors of the present invention developed a surface acoustic wave resonator using a surface acoustic wave element 20 (Japanese Patent No. 3216137). As shown in FIG. 10, the surface acoustic wave element 20 is obtained by rotating the X and Y′ axes about the Z′ axis by φ° to be a X′ axis and a Y″ axis respectively such that its cutting angle is expressed as (0°, θ°, Ψ°) with the Euler angle. In the surface acoustic wave element 20 formed by rotating the ST cut quartz plate 10 in the plane, an IDT 14 and reflectors 16 are formed along the X′ axis. This makes it possible to make its second-order temperature coefficient smaller than that of the surface acoustic wave element 12 formed without rotating the ST cut quartz plate 10 in the plane, thereby resulting to extremely excellent frequency characteristics.
However, in the surface acoustic wave element 20 formed by rotating the ST cut quartz plate 10 in the plane, the direction of the phase velocity that a phase of the surface acoustic wave is propagated and the direction of the group velocity that a wave group (wave packet) is propagated are different because quartz single crystals have anisotropy (“Surface Acoustic Wave Element Technical Handbook”, Edited by Japan Society for the Promotion of Science Surface Acoustic Wave Technical Committee 150, First Edition First Printed, Issued 1991, Nov. 30, p 154). The direction of the group velocity is the direction that energy of the surface acoustic wave is propagated. The angle formed between the direction of the phase velocity and the direction of the group velocity is called a power flow angle PFA. In addition, the surface acoustic wave element cannot obtain a large Q value required for resonators unless the wave group propagated in the direction of the group velocity is reflected. Accordingly, in a case where the IDT and the reflectors are formed along the X′ axis, the surface acoustic wave generated in the IDT is not efficiently reflected by the reflectors. As a result, this lessens the Q value. Therefore, the inventors of the invention developed, in Japanese Patent No. 3216137, a surface acoustic wave element 30 shown in FIG. 11. In the surface acoustic wave element 30, a IDT 34 and reflectors 36 provided both side of the IDT 34 that are formed on a surface of a quartz plate 32 are arranged along the direction of the group velocity so as to reflect the surface acoustic wave by the reflectors 36 to obtain a large Q value.
Meanwhile, according to “Surface Acoustic Wave Element Technical Handbook”, Edited by Japan Society for the Promotion of Science Surface Acoustic Wave Technical Committee 150, First Edition First Printed, Issued 1991, Nov. 30, p 154, a power flow angle PFA of an in-plane rotated quartz plate, namely, the PFA that is the angle formed between the direction of the phase velocity and the direction of the group velocity of the surface acoustic wave, can be obtained by the equation 1.PFA=tan−1{(1/v)·(∂v/∂Ψ)}  Equation 1
where v is the phase velocity of the surface acoustic wave and Ψ is the in-plane rotated angle of the quartz plate.
However, conventionally, an accurate phase velocity of the surface acoustic wave generated in the in-plane rotated ST cut quartz plate has not been obtained. Thus, it is uncertain that whether or not the IDT and reflectors arranged in the direction of the group velocity based on the power flow angle PFA obtained by the equation 1 is really arranged in the direction of the group velocity.
Also, the surface acoustic wave element is generally formed a rectangular like.
Similarly, in the surface acoustic wave element 30 described in Japanese Patent No. 3216137, the quartz plate 32 on which the IDT 34 and a pair of reflectors 36 are formed is cut as a rectangular like from the quartz wafer. Then, the quartz plate 32 is cut such that two edges faced each other being long edges 38 and 39 are along the X′ axis that is the direction of the phase velocity of the surface acoustic wave. Because of this, in the surface acoustic wave element 30, wasted regions are formed both sides of the IDT 34 and the reflectors 36. Thus, the area of the quartz plate 32 becomes unnecessarily larger than the region in which the IDT 34 and the reflectors 36 are formed and the surface acoustic wave is effectively propagated. Therefore, the surface acoustic element 30 becomes so large that the number of elements formed in one wafer and a yield rate lessen. As a result, costs are increased.
In order to solve the disadvantages of the above-mentioned related art, the present invention aims to enable the IDT and reflectors to be reliably arranged along the direction of the group velocity of the in-plane rotated quartz plate.
Also, the invention aims to enable the Q value of the surface acoustic wave element using the in-plane rotated quartz plate to be large.
Further, the invention aims to enable the surface acoustic element to be smaller.
Moreover, the invention aims to enable the yield rate to be increased.